Nitride semiconductor substrate

ABSTRACT

In an independent GaN film manufactured by creating a GaN layer on a base heterosubstrate using vapor-phase deposition and then removing the base substrate, owing to layer-base discrepancy in thermal expansion coefficient and lattice constant, bow will be a large ±40 μm to ±100 μm. Since with that bow device fabrication by photolithography is challenging, reducing the bow to +30 μm to −20 μm is the goal. The surface deflected concavely is ground to impart to it a damaged layer that has a stretching effect, making the surface become convex. The damaged layer on the surface having become convex is removed by etching, which curtails the bow. Alternatively, the convex surface on the side opposite the surface having become convex is ground to generate a damaged layer. With the concave surface having become convex due to the damaged layer, suitably etching off the damaged layer curtails the bow.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to methods of manufacturing reduced-bownitride substrates for semiconductors, and to nitride semiconductorsubstrates manufactured by the method.

Substrates on which semiconductor devices are fabricated are roundwafers, and given that the devices are fabricated on the front surfaceof the substrates by such methods as photolithography, doping,diffusion, and vapor deposition including chemical vapor deposition(CVD), the front surface must be flat, with minimal bow. Whenfabricating semiconductor devices onto silicon and onto gallium arsenidein particular as substrates, Si and GaAs wafers with minimal bow,polished to an optically smooth, mirror finish are employed.

Sapphire wafers are used as the substrates for blue light emittingdiodes in which indium gallium nitride is the light-emitting layer.InGaN/GaN-based LEDs formed onto sapphire substrates have performed welland are dependable. The sufficiently moderate cost of sapphiresubstrates has meant that InGaN-based LEDs can be made at low-cost.

Nevertheless, there are drawbacks to sapphire. For one, with sapphirebeing an insulator, rather than attaching n electrodes to the bottom, aGaN layer onto the surface of which the n electrodes are attached isapplied, thus requiring excess area. Another is that since sapphire doesnot cleave, it cannot be rent into chips along natural cleavages. Andbecause it is GaN and InGaN that are grown onto the heterosubstratethere is misfit, which leads to heavy defects.

Under the circumstances, then, it is desirable that GaN itself be thesubstrate. GaN substrates have become producible by depositing a thickGaN film onto a heterosubstrate base using vapor-phase deposition andremoving the base to create a GaN freestanding layer. And in terms ofsize, 50-mm diameter substrates—long-awaited—have also become possible.

Vapor-phase grown GaN-crystal wafers are, however, used as-grown forepitaxial deposition substrates. In the front side of GaN substratesthat have been vapor-phase deposited and nothing more roughness isconsiderable and bow is serious; growing GaN and InGaN onto suchsubstrates will not necessarily lead to a lowering of defects over thesituation with sapphire substrates. And LEDs created experimentally onas-grown GaN substrates certainly do not perform better than LEDsmanufactured on sapphire.

Because the formation of semiconductor devices onto GaN substrates is byphotolithography, flat, mirror-finish wafers with minimal bow aredesired as the substrates. Polishing and etching technology is necessaryto render the surface of a wafer optically smooth. Polishing and etchingtechnologies have already been established for fully developedsemiconductor substrates such as Si and GaAs. Si and GaAs crystal can begrown by gradually solidifying a melt, as in the Czochralski method orthe Bridgeman method. Since long, columnar ingots with few dislocationscan be produced by growing from the liquid phase, the ingots are slicedwith an internal-diameter saw to produce wafers. This means that bow isminimal from the start.

With GaN on the other hand, growth, being impossible from the liquidphase, is by means of vapor-phase deposition. Furthermore, what formoptimal polishing and etching methods should take is still notunderstood. If GaN is to be hetero-deposited onto crystal of a differentkind, such as has three-fold symmetry, the growth will necessarily bec-axis oriented. The surfaces are a (0001) plane and a (000 1) plane.Because GaN crystal does not have reverse symmetry, the (0001) and(000 1) planes are not crystallographically equivalent. The (0001) faceis one in which gallium atoms range in lines globally over theepisurface, and the (000 1) face is one in which nitrogen atoms range inlines globally over the episurface.

The former can be referred to as the (0001) Ga face, or simply the Gaface; the latter, as the (000 1) N face, or simply the N face.Physiochemically the Ga face is extremely unyielding and rugged, and isnot dissolved by chemical agents. The N face is also physiochemicallyrobust, but is corroded by certain types of strong acids and alkalis.GaN crystal has such asymmetry.

When GaN is grown onto a base substrate, the front side and back sidebecome either the Ga face or the N face. Depending on how the basesubstrate is selected, the front side can be made the Ga face or the Nface. The back side then becomes the face with the opposite polarity.

For the sake of simplicity, a case in which the front side is the (0001)Ga face, and the back side is the (000 1) N face will be considered. Thesame statements can be made, and the same design features implemented inthe opposite situation as well.

Since the subject of the present invention is bow, to begin with adefinition of bow will be given. bow can be expressed as radius ofcurvature, or curvature. These are exact expressions and can be givenlocally. Even in situations in which the bow is complex and thesubstrate has heavy roughness, exact bow can be expressed using a localcurvature expression. For example, bow with a saddle point andcylindrical-lens-like bow can also be expressed.

But with uniform buckling in round wafers, bow is often represented by asimpler expression. If the roughness is uniform the wafer is measuredtaking the height H to the planar face from the surface of the centerarea in the convexity, according to which a value for the bow is given.This is intuitive, and facilitates measurement. The absolute value isdetermined by this bow measurement.

The sign of the bow will be given by its orientation. This definition isindicated in FIG. 1. bow curving outward along the front side will betermed positive (H>0); bow curving inward along the front side will betermed negative (H<0).

In situations in which long monocrystal ingots with few dislocations canbe produced—such as is the case with Si and GaAs—since the ingots aresliced with an internal-diameter saw or a wire saw, bow is slight fromthe start. To produce GaN crystal, however, with growth from the liquidphase being impossible, vapor-phase growth is carried out. Becauserendering GaN crystal is by heteroepitaxy onto a heterosubstrate thatdiffers from GaN in thermal expansion coefficient, and then removal ofthe heterosubstrate, considerable bow appears in GaN crystal. Thisproblem is due not only to the difference in thermal expansioncoefficient, but also to the many dislocations that come about becausethe base substrate and the overlying film are different materials. Thedislocations give rise to irregular stresses, which due to the volume ofdislocations is why bow comes about.

As-grown, platelike, 20-50 mm diameter GaN crystal from which the basesubstrate has been removed has a bow of from ±40 μm to as much as ±100μm, although the value will differ depending on the type andcrystal-plane orientation of the base substrate, and on the vapor-phasedeposition parameters.

With the bow in a GaN wafer substrate being that extensive, in asituation in which a photolithographic resist on the wafer is to beexposed its dimensions will be thrown out of balance. Thus the bow mustbe extensively reduced. bow in Si and GaAs wafers also has to belessened, but with GaN there is a special reason why bow has to bereduced. Since GaN is transparent, when the wafer is set on a susceptorwith a built-in heater and heated, not much of radiant heat from theheater heating the GaN crystal occurs. Seeing as how thermal conductionfrom the susceptor is the principal heat-transmission means, the backside of the GaN crystal desirably is flat, with its entire surface incontact with the susceptor without gaps.

Instances of the above outward-curving (positive bow, H>0) mean that thewafer center portion comes apart from the susceptor. Such cases arestill the better, because the thermal conduction is from the peripheralmargin heading toward the center. Oppositely, in instances of the aboveinward-curving (negative bow, H<0), with only the center contactingsusceptor the wafer ends up turning, leading to positional instability.Not only that, but source-material gases circle to the back side throughthe encompassing, lifted-up area, causing thin-film growth or etching tooccur on the backside of the substrate also. Consequently, negative bowis even less suited to semiconductor fabrication needs than positivebow.

Because as-grown GaN crystal has a bow H of from ±40 μm to ±100 μm, thenumber one objective is to decrease the bow to be within a +30 μm to −20μm range.

More advantageously, the bow should be decreased to within a +20 μm to−10 μm range.

Furthermore, if possible, bringing the bow to within +10 μm to −5 μmwould even better meet fabrication needs.

There are any number of examples of devising a crystal growth method tominimize bow in the products. These may be grossly bifurcated into thosethat reduce bow by lateral overgrowth of the GaN to alleviate verticallyoriented stress and reduce internal stress, and those that grow twolayers having competing actions and eliminate bow by the balance betweenthe actions. Every one of these is a way of attempting to reduce, viathe deposition parameters, bow in crystal being grown; they are not waysof attempting to reduce bow in crystal already produced.

2. Description of the Related Art

Japanese Unexamined Pat. App. Pub. No. H11-186178 addresses the problemof incidents of bow and cracking in GaN crystal that due to thedifference in the coefficients of thermal expansion of Si and GaN occurwhen a GaN film is grown onto an Si substrate to create a GaN/Sicomposite substrate.

This reference relates that to prevent bow and cracking from occurringin GaN crystal, stripes of SiO₂ film are formed onto an Si substrate,and when GaN film is grown onto the substrate, atop the SiO₂ growth ofGaN does not initially occur, thereby alleviating stress and reducingbow in the GaN/Si composite substrate. This substrate is not anindependent film of GaN, but rather a composite substrate in which athin GaN layer on the order of 10 μm is provided on an Si base, so thatinternal stress in the GaN layer can be reduced by having the SiO₂intervene.

Japanese Unexamined Pat. App. Pub. No. 2002-208757 concernsmanufacturing nitride semiconductor substrates of satisfactorycrystallinity, by employing lateral overgrowth and, to keep bow undercontrol, dispersing throughout the substrate overall the coalescenceboundaries, where defects concentrate.

Japanese Unexamined Pat. App. Pub. No. 2002-335049 proposes a depositionmethod that by reducing dislocations by means of lateral overgrowth todiminish stress, also reduces bow.

Japanese Unexamined Pat. App. Pub. No. 2002-270528 proposes a depositionmethod in which reducing dislocations by means of lateral overgrowth toreduce stress keeps bow from occurring.

Japanese Unexamined Pat. App. Pub. No. 2002-228798 exploits Si crystalnot as a semiconductor but as a mirror. The goal is to create concave orconvex mirror surfaces from Si crystal. To get Si crystal to possess adesired curvature, it must be deformed. To do so, a thin film of diamondis built up on an Si substrate, and the Si substrate is deformed by thestress between the diamond thin film/Si substrate. In other words, theoriginal planar article is forcibly buckled to lend it a concave orconvex mirror surface. The reference states that Si can be buckled intoa curvature of choice depending on the diamond formation parameters.

Japanese Unexamined Pat. App. Pub. No. 2003-179022 addresses the problemthat after forming semiconductor devices onto a large-caliber Si wafer,the wafer is back-side ground and the back side is mechanically planedto reduce the wafer to a desired thickness, but a processing distortionlayer is formed, producing a bow of 800 μm, and etching away the layertakes too much time. This reference states that, given the realizationthat the processing distortion layer on the Si wafer back side isamorphous, bow is eliminated by exposing the Si back side for 5 secondswith light from a halogen lamp to momentarily heat the wafer to 600-700°C. and convert the processing distortion layer from an amorphous to acrystalline state. Thus this is an example not of ridding the wafer ofthe processing distortion layer, but eliminating bow in the wafer byqualitatively transforming the layer.

Inasmuch as nitride semiconductor is chiefly produced using vapor-phasedeposition to build up a thin film onto a heterosubstrate and removingthe base substrate, with dislocations due to the difference in thermalexpansion coefficients and the mismatching lattice constants occurringat a high density, bow is serious. Although methodologies fordiminishing bow by devising growth methods to diminish internal stresshave been variously proposed, they are yet insufficient.

Even with such methodologies, manufacturing nitride semiconductorcrystal of large film thickness and large diameter means thedislocations and bow will be considerable, and when the base substrateis removed the crystal often ends up cracking. Even if the crystal doesnot crack, the bow will be large, reaching ±40 μm to as much as ±100 μm.

BRIEF SUMMARY OF THE INVENTION

Objects of the present invention are in such crystal substrates in whichbow is large to reduce the bow by means of a post-deposition process.

A first object is bringing out a processing method so that the bowfigure for nitride semiconductor substrate as 2-inch wafers is broughtto within a range of +30 μm to −20 μm. A second object is bringing out aprocessing method that brings the bow figure for GaN substrates towithin +20 μm to −10 μm. A third object of the present invention ismaking available a processing method in which, the bow figure fornitride semiconductor substrates is reduced to within +10 μm to −5 μm bymeans of a post-deposition process. A fourth object of the presentinvention is bringing out nitride semiconductor substrates in which thebow is within +30 μm to −20 μm.

A method of manufacturing nitride semiconductor substrates according toone aspect of the present invention addresses bow in a nitridesemiconductor substrate by mechanically grinding, to introduce a damagedlayer into, the concave face of the buckled substrate, thereby extendingthe concave face, bringing it close to being planar and reducing thebow.

In accordance with a nitride substrate manufacturing method in anotheraspect of the invention, by mechanically grinding, to introduce adamaged layer into, the concave face of a nitride semiconductorsubstrate in which there is bow, the concave face is extended to deformit convexly; and by etching the convexly deformed surface to remove thedamaged layer partially or entirely and bring down the convex face, thesubstrate is brought close to being planar, which reduces the substratebow.

According to a manufacturing method in a further aspect of theinvention, by mechanically grinding, to introduce a damaged layer into,the concave face of a nitride semiconductor substrate in which there isbow, the concave face is extended to deform it convexly; the convexlydeformed surface is etched to remove the damaged layer partially orentirely and bring down the convex face; and by mechanically grinding,to introduce a damaged layer into, the surface that has turned into aconcave face on the opposite side, the concave face is extended,rendering it a convex face; by etching the surface that has now beenconvexly deformed and bringing down that convex face, the substrate isbrought close to being planar, which reduces the substrate bow.

A further aspect of the invention is a manufacturing method according towhich, by mechanically grinding, to introduce a damaged layer into, theconcave face of a nitride semiconductor substrate in which there is bow,the concave face is extended to deform it convexly; and by mechanicallygrinding, to introduce a damaged layer into, the surface that has turnedinto a concave face on the opposite side, the concave face is extended,rendering it a convex face; by etching the surface that has now beenconvexly deformed and bringing down that convex face, the substrate isbrought close to being planar, which reduces the substrate bow.

From the following detailed description in conjunction with theaccompanying drawings, the foregoing and other objects, features,aspects and advantages of the present invention will become readilyapparent to those skilled in the art.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is exaggerated, outline sectional views of a substrate,representing definitions of the sign given to bow, in which convex bowalong the front side is positive, and convex bow along the backside isnegative.

FIG. 2 is a graph plotting measurements of front-side roughness (Ra: μm)and damaged layer depth for when the front side of a 2-inch GaN waferunderwent a grinding operation with #80, #325, and #1000 diamond grit.The horizontal axis is the grit (mesh) number, the right vertical axisis the level of surface roughness Ra (μm), and the left vertical axis isdamaged layer depth (μm). It is apparent from the graph that with thegrit as the mediating agent, the deeper the damaged layer is, the largerthe surface-roughness level becomes.

FIG. 3 is a graph plotting measured values of bow H against those ofetching depth when a damaged layer on the back side (N face) of apost-back-side-ground GaN wafer was wet-etched utilizing a KOH solvent.The horizontal axis is the etching depth (μm), and the vertical axis isthe wafer bow H (μm). From the graph it is evident that etching a waferwith an initial −33 μm concave bow (curving inward along the front side)proceeded to decrease the bow. When some 5 μm had been etched, the bowbecame a constant—10 μm or so, not decreasing to less than that.

FIG. 4 is a graph plotting measured values of bow H against those ofetching depth when a damaged layer on the front side (Ga face) of a GaNwafer after having been ground utilizing a chlorine plasma wasdry-etched. The horizontal axis is the front-side etching depth (μm),and the vertical axis is the wafer bow H (μm). From the graph it isevident that etching a wafer with an initial +41 μm convex bow (curvingoutward along the front side) proceeded to decrease the bow. When some 6μm had been etched, the bow became a constant +10 μm or so, notdecreasing to less than that.

FIG. 5 is crystal-section views for explaining fundamental techniques ofthe present invention for reducing bow in wafers by combining formationof a damaged layer by grinding, and reduction of the damaged layer byetching. The upright lines represent dislocations, and the specklesrepresent damaged layers. FIG. 5A illustrates a technique for asituation in which a post-growth substrate crystal is convexly buckledalong the front side (H>0), in which grinding the back side creates adamaged layer on the back side, extending the back side and reducing thebow. FIG. 5B illustrates a technique for a situation in which apost-growth substrate crystal is concavely buckled along the front side(H<0), in which grinding the front side introduces a damaged layer onthe front side, extending the front side and reducing the bow. FIG. 5Cillustrates a technique for a situation in which grinding the back sidehas produced a damaged layer to excess, resulting in concave bow alongthe front side, in which the damaged layer on the back side is removedby etching, which thins down the damaged layer to reduce the bow.

DETAILED DESCRIPTION OF THE INVENTION

From stages in manufacturing a GaN substrate to grinding and etching inthe present invention will be explained in more detail.

1. Growing GaN Ingots

GaN freestanding layers are created according to the method set forth inJapanese Unexamined Pat. App. Pub. Nos. 2000-12900 and 2000-22212. Anepitaxial lateral overgrowth (ELO) mask is laid onto a (111) GaAs wafer,and the GaN is grown by a vapor-phase epitaxy technique such as hydrideor metalorganic-chloride vapor phase epitaxy (HVPE or MO-chloride VPE).

The GaN is grown on the ELO mask to reduce stress in the crystal, andmoreover is subjected to facet growth to reduce dislocations. Thedeposition yields GaN of 100 μm to several mm in thickness, and the GaAssubstrate is removed to give an independent GaN substrate.

Techniques for removing the GaAs base substrate include dissolving withaqua regia, shaving off by polishing, and delaminating by a lift-offprocess. GaN films grown thin render single, freestanding GaN wafers;when thick they are cut with a wafer saw to yield a plurality of wafers.

As-grown GaN crystal after the GaAs has been removed is often convexalong the back side, and the bow amplitude H is often ±40 μm to as muchas ±00 μm. The roughness (R_(max)) along the back side can be 10 μm ormore. Such serious bow occurs owing to the large difference in thermalexpansion coefficient between the base substrate and the GaN, and to themassive dislocations produced by their mismatching lattices. Occurrencesof such bow are inevitable despite a mask-utilizing lateral overgrowthtechnique as just discussed being carried out.

To have the GaN substrates be as they should for manufacturingsemiconductor devices onto them, the bow must be decreased, and thefront and back sides planarized (lowering the degree of surfaceroughness). Henceforth a discussion of the present invention willdevelop.

2. Evaluating Damaged layer in Ground Substrates

The post-grinding damaged layer on the substrates was evaluated bycross-sectional observation using scanning electron microscopy (SEM) andcathode luminescence (CL).

From the observation results, it was evident that on a substrate inwhich the GaN crystal face was ground employing #325 diamond grit, thedepth of the damaged layer was approximately 4.8 μm.

The mesh (size) of the diamond grit correlates with the surfaceroughness. The rougher the grit is the rougher the surface ground withthe grit will be. With finer grit texture, the face ground with the gritwill turn out planar. In turn, since the damaged layer arises fromgrinding, the damaged layer ought to bear a relationship to theroughness of the grit. This means that by way of the roughness of thegrit texture, there ought to be a correlation between the thickness ofthe damaged layer and the surface roughness.

Given these considerations, the relation between the depth of thedamaged layer and the surface roughness was investigated. The resultsare shown in the FIG. 2 graph. The horizontal axis is the mesh (#). Thelarger the number, the finer the grit is. Plotted in the graph aredamaged layers on GaN crystal planed with #80, #325 and #1000 grit,versus roughness. The vertical axis on the left indicates damaged layerdepth (thickness in μm), while the vertical axis on the right indicatessurface roughness Ra (μm).

From the graph it will be understood that the lower the surfaceroughness, the thinner will be the damaged layer. The depth of thedamaged layer is dependent on the grain size of the diamond gritemployed. The significance of this is that the depth of the damagedlayer can be controlled. Using a fine-textured grit diminishes thedamaged layer and makes it smooth. By the same token, using acoarse-textured grit allows a thick damaged layer to be createddeliberately.

Grinding with a grit of a suitable texture smoothes, and produces adamaged layer on, the GaN substrate face. The damaged layer acts tostretch the surface on which it is formed. If the action is excessive,the crystal will end up buckling oppositely. In order to rectify this,the damaged layer should be partially removed, and to do so etching wascarried out. For the etch, both wet etching using chemical agents anddry etching using plasma were tried.

3. Study of Front-Side Wet Etching

After being processed, the surface of a GaN substrate underwent wetetching. KOH (aqueous solution, 8 N concentration) was heated to 80° C.,and the GaN substrate was wet-etched by immersing it into the solution.The bow was not, however, altered. This means that a GaN crystal face onwhich a damaged layer has been produced by polishing is not wet-etchedby KOH.

The (0001) faces of GaN have polarity. One face (the Ga face) isterminated with gallium atoms, and the other face (the N face) isterminated with nitrogen atoms. The Ga face is hard and unyielding, andis chemically stable. No chemical agent that can effectively etch a Gaface exists. Since the front side was the Ga face and the back side wasthe N face, when the substrate was dipped into the KOA solution theback-side N face was slightly etched but the front-side Ga face was notetched at all. Because the front side, being polished, had the damagedlayer, KOH did not remove the front-side damage layer.

Wet-etching GaN with a strong alkali like heated KOH, or a strong acidsuch as H₃PO₄ has been documented. But such instances have amounted onlyto erosion of the N face. The GaN that the present invention inventorsmanufacture possesses a composite front side in which the N face and theGa face appear in alternation. Since wet-etching the GaN in an etchantsuch as KOH or H₃PO₄ etches only the N face, creating pits, the frontside ends up being ragged. Despite the pains taken to polish the frontside, it ends up ruined, not amounting to anything. Ultimately,therefore, wet-etching of the front side (Ga face) proves to beimpossible.

4. Back-Side Wet Etching

The back side (N face) of GaN substrates is ground. A damaged layer iscreated on the back side by polishing, and the substrates buckleconvexly along the back side (bow: negative). It was discovered thatwhen substrates having a negative bow are wet-etched with an 8 N KOHsolution at 80° C. or with H₃PO₄ phosphoric acid, with elapsed etch timethe absolute value of the bow decreases. That is, the back side—beingthe N face—is etched by a strong alkali and a strong acid, and by thevery diminishment of the diminishing damaged layer, the bow iscurtailed. This means that back-side polishing and wet etching form amethod that can be utilized to curtail bow.

Results of thus utilizing the method are shown in FIG. 3. Underconditions for back-side wet etching identical to those just noted, theback side of a GaN substrate was wet-etched. The horizontal axis in thegraph represents the wet-etching depth (μm), and the vertical axis, bow(μm). From the graph it is evident that wet-etching a concave GaNsubstrate whose front side possessed an initial −33 μm concave bowcurtailed the bow. When some 5 μm had been etched, the bow went toaround −10 μm; etching beyond that did not lead to diminishment of the−10 μm bow.

In addition, variation in the thickness was under several μm, which wasat the non-problematic level.

Wet-etching the back side of the substrate gave the GaN crystal—whosefront side, being globally mirror-finished, was transparent—a cloudedappearance like frosted glass. This was because the back side had beensurface-roughened. Since the bow was reduced, in situations in which itis acceptable for the back side to be glazy, the substrate can be usedin that state.

There are situations, however, in which the back side being glazy wouldcreate problems—in which the back side has to be a mirrorlike surface.In such cases, arrangements have to be made to remove the damaged layerby dry-etching the back side. When removal is by dry etching, the backside does not become frosted-glasslike.

The fact that wet-etching the Ga face is impossible, while wet-etchingthe N face is possible has been noted. The N face (back side) can be ridof a damaged layer by either wet etching or dry etching. For the frontside, removal is only by means of dry etching.

5. Study of Front-Side Dry Etching

Inasmuch as wet etching is ineffectual, the only option for etching thefront side (Ga face) is by dry etching. Provided that dry etching isfeasible, by that means removing a damaged layer along the front side ofa GaN substrate ought to be possible.

Performing dry etching of GaN under the following conditions makes itpossible to etch the front side.

Equipment: reactive ion etcher Gas: halogen gas (chlorine gas) Chlorineflow rate: 5 sccm to 100 sccm Pressure during etch: 0.1 Pa to 10 PaPlasma power: antenna - 100 W to 500 W bias - 5 W to 20 W

Plotted in FIG. 4 is the relationship between front-side etching depthand bow when the front side (Ga face) of a GaN substrate was dry-etchedat: chlorine flow rate=10 sccm; pressure=1 Pa; antenna power 300 W; bias10 W. The horizontal axis is the etching depth (μm); the vertical axisis the bow (μm). Although the bow was initially 40 μm, the etchingcarried out proceeded to curtail the bow: When the etching depth was 0.8μm, the bow had decreased to +30 μm; at 1.3 μm etching depth the bow haddecreased to +22 μm; at 2 μm etching depth, the bow had subsided to +16μm; at 3.6 μm etching depth, the bow had subsided to +13 μm; at 5.5 μmetching depth, the bow had curtailed to +10 μm; and when the etchingdepth had gone to over 6 μm, the bow no longer subsided, staying at the+10 μm level.

It was realized that although with the front side being the Ga face, thefront side could not be etched by wet etching techniques, with a dryetching technique—reactive ion etching (RIE)—the Ga face too could beetched. Then it was also realized that by means of the etching, positivebow (convexity in the front side) decreases. This was a crucialdiscovery. With the damaged layer being on the front side, the layerbrought about positive bow (convexity along the front side). Since whatgave rise to the positive bow was curtailed because the front side wasreduced, the bow proceeded to decrease. Such is the plausibleinterpretation.

6. Study of Back-Side Dry Etching

Under the same conditions as with the front side, dry etching waspossible on the back side (N face) of a GaN substrate. By means of dryetching using chlorine plasma, removal of a damaged layer from the backside was also possible. Removing the damaged layer from the back sidealtered the bow from being concave with respect to the front side tobeing convex with respect to the front side. (The bow changed headingfrom negative-ward to positive-ward.) And removing the damaged layer onthe substrate back side was possible without spoiling the surfacesmoothness of the back side.

7. Controlling Bow

Herein it will become clear that bow can be controlled by combininggrinding or a like mechanical process, and dry etching. A damaged layerforms when either the front side (Ga face) or the back side (N face) isground. The damaged layer produces compressive force on the ground face,tending to stretch it. The front side therefore deflects convexly when adamaged layer is made on the front side. And the back side deflectsconvexly when a damaged layer is made on both sides. The bow rate can bemodulated by the thickness d of the damaged layer, and the damaged layercan be removed by dry etching. If thus the thickness of the damagedlayer is decreased, the bow will change from being convex to beingconcave. These are the reasons why bow can be controlled by theformation of a damaged layer.

Such instances are illustrated in FIG. 5. The plural vertical linesdrawn within the wafers represent dislocations. Further, fine stipplesare drawn by the front/back side of the wafers; these are the damagedlayer produced by grinding. FIG. 5A illustrates a technique for a waferwhose front side is convex (H>0), in which grinding the concave backside creates a damaged layer on the back side to curtail the bow. FIG.5B illustrates a technique for a wafer whose front side is concave(H<0), in which grinding the concave front side forms a damaged layer onthe front side to curtail the bow. FIG. 5C illustrates a technique ofback-side dry-etching in which the back side of a wafer whose front sideis concave (H<0) is ground to create on the back side a damaged layer,and the damaged layer on the back side is reduced and thinned down.

The bow in a GaN substrate deposited by a vapor-phase deposition onto aheterosubstrate, from which the base substrate is removed, is ±40 to asmuch as ±100 μm. If thus the bow is large, the error in the opticalexposure pattern during device fabrication by photolithography will betoo great. When contact exposing a substrate it is pressed upon, and ifthere is bow, the substrate can crack. Therefore, bow in the GaNsubstrate has to be +30 μm to −20 μm. More desirably, the bow is +20 μmto −10 μm, and optimally it is +10 μm to −5 μm.

GaN substrates are transparent. Forming thin films onto the GaN wafersby metalorganic chemical vapor deposition (MOCVD) or molecular-beamepitaxy (MBE), or vapor-depositing electrodes on the wafers means thatthey are placed on a susceptor with a built-in heater and heated; butbecause the wafers are transparent, they do not sufficiently absorb theradiant heat from the heater. Rather than the radiant heat, a waferabsorbs heat from the susceptor due to thermal conduction. Because theabsorption route is by thermal conduction, it is vulnerable to how thewafer and susceptor are in contact. To make the heating uniform, thestate of contact between the wafer and susceptor must be made uniform.If there is bow in the wafer, thermal conduction will be restricted tothe central portion (concave bow) or to the peripheral portion (convexbow). With uniform heating being impossible on account of such bow, astrong, diametrically oriented temperature distribution is set up in thewafer. Consequently, the characteristics of the fabricated devices endup being inconsistent. In this respect GaN substrates differ vastly fromSi and GaAs substrates.

Thus, as far as bow is concerned, more severe conditions are imposed onGaN substrates than on Si or GaAs substrates. Since in order to makethermal conduction uniform, globally even contact with the susceptor issought, zero bow is ideal. The spread in which bow is tolerated is notidentical above and below zero: a tolerance range in which above, wherebow is convex, is up to 30 μm, and below, where bow is concave, is asfar as 20 μm.

Thus the ranges of bow that can be tolerated are

Range (a): +30 μm to −20 μm; Range (b): +20 μm to −10 μm; and Range (c):+10 μm to −5 μm. Equipment: reactive ion etcher Gas: halogen gas(chlorine gas) Chlorine flow rate: 5 sccm to 100 sccm Pressure duringetch: 0.1 Pa to 10 Pa Plasma power: antenna - 100 W to 500 W bias - 5 Wto 20 W

ADVANTAGEOUS FEATURES OF THE INVENTION

If with bow being large semiconductor devices are fabricated byphotolithography onto GaN crystal wafer obtained by using vapor-phasedeposition to grow GaN onto a heterosubstrate and stripping off theheterosubstrate, error in the transfer pattern will be significant. Andthere will be instances of cracking in the wafer when it isvacuum-chucked.

Inasmuch as the present invention brings the wafer bow to within +30 μmto −20 μm, even vacuum-chucked the wafer will not crack. Wafersaccording to the present invention do not fracture even when masks forcontact exposure are set onto the wafers. Since there is no bow, themask pattern is accurately transferred onto the resist, and errors donot appear in the optical exposure pattern. These features improvedevice-fabrication yields.

Inasmuch as a damaged layer is exploited to eliminate bow, the damagedlayer of the present invention remains behind to some extent. A maximumof 50 μm of the damaged layer along the back side, and a maximum of 10μm of the layer along the front side will in some cases be present. Thedamaged layer along the front side is so thin as not to be a hindrancewhen fabricating devices. Even along the back side, since the damagedlayer is 50 μm or less, disruptions, such as growth of cracks orincidents of fracturing, following from wafer-processing basedoperations do not arise.

What the present inventors discovered is that grinding a nitridesubstrate surface with grit having a coarse mesh produces a damagedlayer and the damaged layer has a stretching effect on the surface, andthat by means of etching to diminish the damaged layer this action thattends to stretch the surface is curtailed. Accordingly, a noveltechnique by the present invention is the production of a planarsubstrate with minimal bow by introducing a (grinding) damaged layeronto the front side/back side of a nitride substrate, and removing thelayer in part.

When the bow H is taken into consideration including its sign,front-side damaged layer introduction S and back-side etching T increasethe bow H, while front-side etching U and back-side damaged layerintroduction W decrease the bow H. This means:

H Graduated Increase front-side damaged layer Processes - introductionS, back-side etching T; H Graduated Decrease front-side etching U,back-side process- Processes - transformed layer introduction W.With front-side damaged layer introduction S and front-side etching Ubeing stand-alone processes they do not necessarily have to form a pair.Likewise, with back-side etching T and back-side damaged layerintroduction W being stand-alone processes they do not necessarily haveto form a pair. But because the etching process has to be for removing adamaged layer, front-side damaged layer introduction S has to go aheadof front-side etching U. Likewise, back-side damaged layer introductionW has to precede back-side etching T.

Going a step further, the sign of these processes is taken to expressincrease/decrease in bow. Thus, S and T take positive values; U and Wtake negative values. Since the absolute value of the change in bow dueto etching is smaller than that of change in bow due to a damaged layer,S+T is positive; U+W is negative. That is:S>0; T>0  (1)U<0; W<0  (2)S+U>0  (3)W+T<0  (4)

Letting the initial bow be H_(i) and the final bow be H_(o), thenfundamentallyH _(i) +S+U+W+T=H _(o)  (5)

Ideally the final bow H_(o) is 0, but there is an optimal range about 0,and it is satisfactory to have the range be+30 μm≧H _(o)≧−20 μm  (6)Given the significance of Equation (5), what this means is thatincreasing the bow through front-side grinding (since S is positive),decreasing the bow by front-side grinding (since U is negative),decreasing the bow by back-side grinding (since W is negative), andincreasing the bow by back-side grinding (since T is positive) producesan appropriate (from −20 μm to +30 μm) final bow H_(o). For the sake ofsimplicity, the final bow H_(o) may be conceived of as being 0. Giventhe parameters in Equations (1) through (4), no matter what the initialbow H_(i), it should be possible to bring the final bow to 0, or else towithin the appropriate range (6).

Nevertheless, the fact that the final thickness of the damaged layeralong the front side is 10 μm or less imposes a restriction on S+U(positive value). In turn, the fact that the thickness of the damagedlayer along the back side is 50 μm or less imposes a restriction on W+T(negative value).

Because on W+T can be a negative number whose absolute value isconsiderably large, implementations in which the initial bow H_(i) ispositive mean for the present invention that with the degree of freedombeing especially large, the invention is more easily embodied.

When the initial bow H_(i) is positive—i.e., when there is a convexityalong the front side (Ga face)—then steps S and U can be omitted, andthe bow can be curtailed simply according to(H _(i)>0) H ₁ +W+T=H _(o)  (7)In other words, this means that back-side grinding W and back-sideetching T alone are sufficient. Moreover, if it is the case that changein bow can be accurately controlled by back-side grinding, then theback-side etching T may be omitted. That is, such cases make it that(H _(i)>0) H _(i) +W=H _(o)  (8)This maintains that bow can be eliminated by back-side grinding W alone(Embodiment 3).

In instances in which the initial bow H_(i) is negative—i.e., when thereis a concavity along the front side (Ga face)—then since H has to beincreased, S and T (S, T both positive) are required. But given this,because T necessarily entails W, what can be omitted is only front-sideetching U. Then what is possible in such instances is(H _(i)<0) H _(i) +S+W+T=H _(o)  (9)This states that bow can be curtailed by means of front-side grinding S,back-side grinding W, and back-side etching T alone (Embodiment 2).

Nonetheless, in some cases in which the initial bow H_(i) is negative,using all four steps will be advisable:(H _(i)<0) H _(i) +S+U+W+T=H _(o)  (10)This states that bow can be curtailed by means of front-side grinding S,front-side etching U, back-side grinding W, and back-side etching Talone (Embodiment 1).

Techniques (9) and (10) can be utilized even when the initial bow H_(i)is positive. Accordingly, noting down altogether techniques possible bythe present invention would be as follows.(H _(i)>0) H _(i) +W=H _(o)  (8)(H _(i)>0) H _(i) +W+T=H _(o)  (7)(H _(i) pos./neg.) H _(i) +S+W+T=H _(o)  (9)(H _(i) pos./neg.) H _(i) +S+U+W+T=H _(o)  (10)

EMBODIMENTS

GaN was grown by HVPE onto a GaAs base substrate as described earlier.The GaAs base substrate was removed to render freestanding, independentGaN crystals. The as-grown GaN crystal substrates thus obtained were50.8 mm in diameter (2-inch) and 500 μm in thickness.

The substrates had a concavity along the front side (Ga face), with theabsolute value of the bow being 40 μm or more (H<−40 μm). The surfaceroughness of the front side was R_(max) 10 μm or more. The surfaceroughness and bow were measured employing a stylus surface profilometer(“Surfcom,” manufactured by Tokyo Seimitsu Co.).

The GaN crystals were affixed by means of wax to a platen made ofalumina ceramic, and were then ground under the conditions tabulatedbelow.

TABLE I GaN crystal substrate front-/back-side grinding conditions GaNCrystal Outer diameter: 2-inch (50.8 mm φ); Thickness: 500 μm Grindingsurface (0001) plane; Ga face or else N face Grinding device Rotary-typegrinder Grinding Grit dia.: 200 mm φ parameters Grit/grain size:Diamond, #325 Working revs: 400 rpm Feed rate: 5 μm/min. Grinding slurrysupply rate: 5 L/min.

The planarity (bow) of the GaN crystal substrate still affixed to thepolishing platen immediately after grinding was ±2 μm, and the surfaceroughness R_(max) was 0.5 μm. Because the polishing platen is perfectlyflat, it stands to reason that bow in a substrate bound fast to theplaten will be slight.

The polishing platen was heated to 100° C. to peel the GaN crystalsubstrate off the platen.

The GaN crystal substrate broken away from the polishing platen wasultrasonically cleansed in isopropyl alcohol. bow in the GaN substratein respective stages was then measured.

Grinding as just described was carried out on both the front side (Gaface) and back side (N face).

The grinding produced damaged layers. Arrangements were made to etch thesubstrate so as to remove the damaged layer at once following grinding.Although the N face (back side) could be wet-etched using KOH, on the Gaface (front side), inasmuch as wet etching is ineffectual, dry etchingusing a chlorine plasma was performed. Of course, dry etching the backside also is possible. The etching conditions were:

TABLE II Dry etching parameters Equipment Reactive ion etcher GasChlorine Chlorine flow rate 10 sccm Pressure during etch 1 Pa Plasmapower Antenna: 300 W; Bias: 10 W

Either the front side or the back side may be ground first. ForProcedure A and Procedure B below, the respective sequences areindicated. It is not necessary to set the procedure so that an etchingoperation always follows on a grinding operation; both substrate sidesmay be ground, following which both sides may then be etched (ProcedureC and Procedure D).

Inasmuch as cleaning and drying are performed following the respectivestages, such as when the substrate is broken away from the polishingplaten, and after etching, herein they have been omitted.

Procedure A

-   -   Front-side grinding    -   Front-side dry etch (chlorine plasma)    -   Back-side grinding    -   Back-side wet etch (KOH), or dry etch (chlorine plasma)

The procedural order written out in slightly more detail would be asfollows.

Grow substrate→Affix to platen→Grind front side→Break away from (liftoff of) platen→Dry-etch front side→Affix to platen→Grind back side→Breakaway from (lift off of) platen→Wet-etch or dry-etch back side.

Procedure B

-   -   Back-side grinding    -   Back-side wet etch (KOH), or dry etch (chlorine plasma)    -   Front-side grinding    -   Front-side dry etch (chlorine plasma)

The procedural order written out in slightly more detail would be asfollows.

Grow substrate→Affix to platen→Grind back side→Break away from (lift offof) platen→Wet-etch or dry-etch back side→Affix to platen→Grind frontside→Break away from (lift off of) platen→Dry-etch front side.

Procedure C

-   -   Front-side grinding    -   Back-side grinding    -   Front-side dry etch (chlorine plasma)    -   Back-side wet etch (KOH), or dry etch (chlorine plasma)

Procedure D

-   -   Back-side grinding    -   Front-side grinding    -   Back-side wet etch (KOH), or dry etch (chlorine plasma)    -   Front-side dry etch (chlorine plasma)

In Embodiment 1 set forth below, Procedure A is adopted, with thesubstrate bow being measured in the post-grown free state, in thepost-grinding bound state as adhered to the platen, in the free stateafter being broken away from the platen, in the free state followingfront-side etching, in the bound state as adhered to the platenfollowing back-side grinding, and in the free state following back-side.

Embodiment 1 Concave Bow (H<0): Front-Side Grinding→Front-SideDE→Back-Side Grinding→Back-Side DE

The bow in the free state of a (2-inch φ, 500-μm thickness) GaN crystalfrom which the GaAs base substrate had been removed was H=−50 μm(front-side concavity). The back side was affixed to the polishingplaten and the front side was ground. The grinding conditions were asdescribed earlier. The absolute value of the post-grinding front-sidebow in the GaN crystal as affixed in the bound state was no more than 1μm. The bow in the GaN crystal in the free state as having been liftedoff the platen was H=+30 μm.

This means that along the front side the crystal had gone convex. Thereason for this is because a thick damaged layer had been introducedinto the front side by the grinding, and the damaged layer generatedstress that tended to stretch the front side. Because the presence of adamaged layer on the front side is not acceptable, the front side wasgiven a dry etch (DE) with a chlorine plasma. Thereafter the bow provedto be H=+10 μm. Although the condition of convexity along the front sidedid not itself change, the amount of bow was reduced. In addition, thefront side was affixed to the platen and the back side was ground. Thegrinding conditions were as described earlier. The post-grindingback-side bow in the GaN crystal as adhered fast to the platen was nomore than 1 μm.

The bow in the GaN crystal in the free state as having been undone fromthe platen was −20 μm. The reason for this is because a damaged layerhad been produced along the back side by the grinding, and the damagedlayer acted to stretch that surface. The bow in the free state after theback side next had been dry-etched was H=−5 μm. This means that the bowhad for the most part disappeared. This bow sufficiently satisfiesaccording to the present invention the condition: +30 μm≧H≧−20 μm; itsatisfies the more preferable condition: +20 μm≧H≧−10 μm; and in fact itsatisfies the optimal condition: +10 μm≧H≧−5 μm.

Grinding gives rise to a damaged layer and since the layer pressinglystretches the ground surface, the bow changes to the opposite side. Andthe further significance is that when the damaged layer is removed byetching, the bow is curtailed in correspondence with the amount removed.In sum, what this means is that by combining grinding and etching, thebow can be reduced or eliminated.

TABLE III Embodiment 1 change in warp immediately after crystal growth,after front-side grinding, after lift-off, after front-side dry etch,after back-side grinding, after lift-off, and after back-side dry etchStage Warp H (μm) Just after crystal-growth (free state) −50 Afterfront-side grinding (bound state)  0 After lift-off (free state) +30After front-side dry etch (free state) +10 After back-side grinding(bound state)  0 After lift-off (free state) −20 After back-side dryetch (free state) −5

Embodiment 2 Concave Bow (H<0): Front-Side Grinding→Back-SideGrinding→Back-Side DE

Embodiment 2 is one in which the front-side dry etch (DE) of Embodiment1 was omitted.

The bow in the free state of a (2-inch φ, 500-μm thickness) GaN crystalfrom which the GaAs base substrate had been removed was H=−50 μm(front-side concavity). The back side was affixed to the polishingplaten and the front side was ground. The grinding conditions were asdescribed earlier. The absolute value of the post-grinding front-sidebow in the GaN crystal as affixed in the bound state was no more than 1μm. The bow in the GaN crystal in the free state as having been liftedoff the platen was H=+30 μm.

This means that along the front side the crystal had gone convex. Thereason for this is because a thick damaged layer had been introducedinto the front side by the grinding, and the damaged layer generatedstress that tended to stretch the front side. No dry etch was performedon the front side, but the front side was affixed to the platen and theback side was ground. The grinding conditions were as described earlier.In the back-side grinding there were instance in which local crackingoccurred. The post-grinding back-side bow in the GaN crystal as adheredfast to the platen was no more than 1 μm.

The bow in the GaN crystal in the free state as having been undone fromthe platen was −30 μm. The reason for this is because a damaged layerhad been produced along the back side by the grinding, and the damagedlayer acted to stretch that surface. The back side was next dry-etched.Thereafter the bow in the free state was H=−20 μm. This bow satisfiesaccording to the present invention the condition: +30 μm≧H≧−20 μm. Thisis a bow range within which photolithography is possible. Of particularsignificance here is that because front-side etching was not carriedout, a factor that makes H positive was diminished.

TABLE IV Embodiment 2 change in bow immediately after crystal growth,after front-side grinding, after lift-off, after back-side grinding,after lift-off, and after back-side dry etch Stage Bow H (μm) Just aftercrystal-growth (free state) −50 After front-side grinding (bound state) 0 After lift-off (free state) +30 After front-side dry etch (freestate) — After back-side grinding (bound state)  0 After lift-off (freestate) −30 After back-side dry etch (free state) −20

Embodiment 3 Convex BOW (H>0): Back-Side Grinding

The bow in the free state of a (2-inch φ, 500-μm thickness) GaN crystalfrom which the GaAs base substrate had been removed was H=+30 μm(front-side convexity). The crystal was affixed to a ceramic platen, andboth sides were ground so as to lessen the damaged layer. That meant afine-mesh grit was employed. The Ra was not more than 5 nm.

In this embodiment, bow could be eliminated without creating afront-side-ground damaged layer and without etching, which was simpler.The front side was affixed to the polishing platen and the back side wasground. The grinding conditions were as described earlier. Thepost-grinding back-side bow in the GaN crystal as adhered fast to theplaten was no more than 1 μm. The bow in the GaN crystal in the freestate as having been lifted off the platen was +10 μm. Because the bowwas “+,” back-side etching was not performed. Significant in thisembodiment—an instance in which the bow was convex—is that the bow couldbe curtailed simply by introducing a damaged layer into the back side.

TABLE V Embodiment 3 change in bow immediately after crystal growth,after back-side grinding, and after lift-off Stage Bow H (μm) Just aftercrystal-growth (free state) +30 After front-side grinding (bound state)— After lift-off (free state) — After front-side dry etch (free state) —After back-side grinding (bound state)  0 After lift-off (free state)+10 After back-side dry etch (free state) —

Herein, should the bow be negative after the back side is ground(convexity along back side), etching the back side to take away part ofthe damaged layer will bring the surface closer to planar (H→0).

Only selected embodiments have been chosen to illustrate the presentinvention. To those skilled in the art, however, it will be apparentfrom the foregoing disclosure that various changes and modifications canbe made herein without departing from the scope of the invention asdefined in the appended claims. Furthermore, the foregoing descriptionof the embodiments according to the present invention is provided forillustration only, and not for limiting the invention as defined by theappended claims and their equivalents.

1. A freestanding nitride semiconductor substrate of approximately 20 to50 mm diameter and 500 μm thickness, the substrate having amixed-polarity front side in which Ga-polar and N-polar faces appear inalternation, and having present on the substrate backside a damagedlayer of greater than 0 μm and less than or equal to 50 μm in thickness,whereby the substrate has a bow H satisfying +30 μm≧H≧−20 μm, wherein,letting the sign of convex bow along the Ga face be positive and thesign of concave bow along the Ga face be negative, the bow H is definedas the magnitude, with the applicable sign affixed, of the distancebetween a plane in which the circumferential margin of the substratelies, and the substrate center point.
 2. A nitride semiconductorsubstrate as set forth in claim 1, wherein +20 μm≧H≧−10 μm.
 3. A nitridesemiconductor substrate as set forth in claim 1, wherein +10 μm≧H≧−5 μm.4. A nitride semiconductor substrate as set forth in claim 2, whereinthe front side is a (0001) plane and the backside is an N-polar (0001)plane.
 5. A nitride semiconductor substrate as set forth in claim 3,wherein the front side is a (0001) plane and the backside is an N-polar(0001) plane.
 6. A nitride semiconductor substrate as set forth in claim1, having present on the substrate frontside a damaged layer of greaterthan 0 μm and less than or equal to 10 μm in thickness.